The present invention relates to a method of determining a draft schedule for a continuous rolling mill, and in particular to a method which permits target values for the crown and flatness of finished sheets to be met.
Japanese Patent Laid Open No. 55-81008 discloses a method of determining a draft schedule for a continous rolling mill in which consideration is given to sheet crown and flatness. In that method, in accordance with the sheet dimensions, the minimum entry side sheet thickness such that the entry side crown will not influence the exit side crown is determined. For the first rolling stand having an entry side thickness less than this minimum entry side thickness and for each stand to the rear of that stand, a crown prediction formula and a rolling load prediction formula are established based on finished sheet target thickness and finished sheet target crown. Using these formulas, the entry side thickness is computed for each stand in succession, using a constant crown ratio for each stand.
However, as can be seen by reference to Equations (2), (4), and (5) explained hereinbelow, if as in that method the crown ratio for the rear stands is held constant (if K.sub.i =K.sub.i-1, or in other words flatness X.sub.i =0), since in the early stages of a rolling cycle, the thermal crown and therefore the roll crown C.sub.Ri are small, in order to make the finished sheet crown small it is necessary to make the rolling load F.sub.i small for the rear stands and large for the front stands. However, this can result in a draft schedule in which the rolling load for the front stands exceeds allowable bounds and becomes too large for rolling to be possible. Conversely, in the final stages of a rolling cycle, the thermal crown and therefore the roll crown become large. Therefore, it becomes necessary to make the rolling load small for the front stands and large for the rear stands, and a draft schedule can result in which the rolling load for the rear stands exceeds allowable bounds, making rolling impossible.